Frequency modulation system



Aug. 25, 1942.

H. RODER 2,294,209

FREQUENCY MODULATION SYSTEM Filed Nov. 12, 1938 am wt cos(m 6mm) s p wtcosfmsm Ut)+ cos sm wt .3 w sm(m sm Lit). Fig. I.

F|L"l'E R 9 I I AMPLITUDE 4 3 HODULATORS l PHASE smFT T NETWORKH cos(msm Ht). A

cos gut cos wi'sm (m sm at) Fig.4. 7

1.. llMlllfllllMllllln.

Inventor: Hans Roder His Attovney.

Patented Aug. 25, 1942 UNI ED STATES PATENT =-omce FRE UENCY nzfirxzrrousurfsrap i Schenectady, N. 1., assignor a Gen eral Electric Company, acorporation of New York - Application November 12, 1938, Serial a...240,145

My invention relates to signaling systems, and more particularly to suchsystems employing socalled, phase, or frequency modulation, of a carrierwave, that is, modulation of the frequency of the carrier wave inaccordance with a signal. It has for one of its objects to provide a newand improved method and means for producing a carrier wave having itsfrequency accordance with the signal.

A further object of my invention is to produce a frequency modulatedcarrier wave without use of the usual frequency modulating means, i. e.,without varying the frequency of any source in accordance with thesignal. n object ofthe invention is to eil'ect frequency modulations bythe use of equipment such' as is commonly used in amplitude modulationsystems.

The novel features which I believe to be char acteristic of my inventionare set forth with par-e ticularity in the appended claims. My inventionitself, however, both as to its organization and method of operationtogether with further objects and advantages thereof may best beunderstood by reference' to the following description taken inconnection with the accompanying drawing in which Fig. 1 represents anembodiment of my invention; Figs. 2 and 3 represent details thereof, andFig. 4 represents a modification.

. Aphase or frequency modulated wave is mathematically represented 'bythe following expresmodulated in v may be represented by the expressionsin at and the oscillations supplied to modulator 4 may be representedby the expression cos wt.

Conductors A and IB,-which comprise the signal input circuit ofmodulator 3 are supplied with currents which may be represented by theex-- pression cos (m sin at), and similarly the conductors C and D whichcomprise the input to. the modulator I is supplied with currents whichmay be represented by the expression sin (m sin at).

These latter currents are of the signal frequency a and modulate thecurrents of the carrier wave frequency supplied tothe respectivemodulators 3 and {to produce, in the output of the modulator 3, acurrent which may be expressed sin wt cos (m sin t) and at the output ofmodulator 4, a

current which may be expressed by cos wt sin rier frequency and theoutput of which is supsion. sin (wt+m sin t). This expression, by

trigonometric methods. may readily be shown to be equal to thefollowing: sin (at cos (m sin ,ut)

cos mt sin I (m sin t). In these expressions w=2wf where I is thefrequency or the carrier wave: 1

t=time l a he signal frequency; and m is a constant for phasemodulation, modulation m=Aw/p where A40 is the maximum shift of thecarrier frequency produced by th signal- Reierring to Fig. 1 I haveindicated at l a source of carrier wave oscillations the output of whichis supplied to two amplitude modulators 3 and plied through a poweramplifier 8 and radiated from an antenna 8. The current in the antennacalled a frequency modulated wave.

The remainder of the apparatus of Fig. 1 comprises means for generatingthe two currents which are supplied to the modulators over con ductorsA, B; and C, D. This means, the apparatus of Fig. 1, comprises a cathoderay tube It and for frequency 4, each of which may be of any well-knowndesign for effecting amplitude modulation of a carrier wave. Theoscillations supplied toamplitude oscillation supplied to modulator 3from source I having therein the usual cathode" II and means whereby abeam 01 electronsis produced and directed upon avspot on a fluorescentscreen at the end I! of the cathode my device. The direction of the beamis controlled by four deflecting plates comprising a pair or horizontalplates It and Il between which oscillations of very high frequency suchas, for example, five megacycles or other suitable frequency much higherthan the signal frequency are supplied from a source It. These platesserve to deflect the beam upwardly and downwardly so as to cause it, inthe absence of signal currents, to produce a luminescent line upon thefluorescent screen of the cathode ray device. The beam is also deflectedhorizontally by means of a pair of electrodes l8 and ll between whichthe signal oscillations are supplied. The source or signal oscillationsis represented as a microphone It the output of which is suppliedthrough an amplifier I 9 and a network comprising a series resistance 23and shunt condenser 2|. In this way the luminescent vertical lineproduced by the source I oi thecathode ray oscillograph is caused todeflect bodily horizontally across the screen in accordance with thesignal currents. Light from the upper portion of the cathode ray screenl2 issuitably focused by means of a lens 22 upon a photoelectric cell 23the output or which is amplified and supplied between the conductors Aand B. Light from the lower portion of the cathode my screen is focusedby means of a lens 25 upon a photoelectric cell 26 the output of whichis amplified by amplifier 21 and supplied between conductors C and D.

In order that light from the upper portion of the cathode ray screen maynot afiect the photoelectric cell 26, and similarly, in order that lightfrom the lower portion of the screen may not affect photoelectric cell23 a suitable screen 28 is provided which extends between the lenses 22and 25 and to a point closely adjacent the middle of the cathode rayscreen. v

0n the end of the cathode ray tube either exteriorly or'interiorlythereof, but between the fluorescent material and the focusing lens is asuitable coating of opaque material having a design suchas is indicatedin Fig. 2 in which the middle portion of the screen is indicated by theline 28, which may be taken to represent the shield 23 of Fig. 1. Theportionof the screen above line 28 affects photoelectric cell 23 andthat below line 23 affects photoelectric cell 26. The shaded area ofboth portions, or sections, as shown in Fig. 2, is covered with opaquematerial so that light from the fluorescent screen within the tube isnot visible exteriorly thereof and only light from that portion of thefluorescent screen not covered, by the opaque material afi'ects thephotoelectric cells.

The vertical line 29 represents the luminescent line which is producedby the cathode ray beam when no signal currents are applied to thehorizontal deflecting plates. When signal currents are applied, thisline 29 moves bodily to the right and to the left, parallel with itself,in accordance with the variations of the signal ductors 36 and 31respectively spaced at regularcurrents. Since the upper portion of eachof the shaded areas are bounded by sinusoidal lines 30 and 3|respectively which are displaced with respect to each other, and withrespect to the midposition of line 29, by 90 degrees, it will be seenthat the light which afi'ects the photoelectric cell 23 varies as acosine function of the signal currents and similarly light from thelower sectlon of the screen which affects photoelectric cell 26varies'as a sine function of the signal current. Accordingly,- currentsrepresenting this cosine function of the signal currents appear uponconductors A and B at the output of amplifier 23 and similarly currentsrepresenting the sine function of the signal currents appear onconductors C and D at the output of amplifier For frequency modulation,as previously stated, 1n. must represent Aer/1L, that is, the ratio ofthe shift in the carrier frequency to the signal frequency. To effectthis it is merely necessary to include the network 20-2! in the outputof the Signal amplifier l9, this network being so proportioned that thevoltage betweenplates I6 and. I1 varies. inversely with the frequency.Thusthe current radiated from the antenna is a ,frequency modulatedcurrent, the modulationbeing in accordance with the current inmicrophone l8.

By the terms phase modulation" and frequency modulation as usedherein, Ido not intend to infer that modulation of the frequency of the carrierwave is not involved in both of these forms of modulation. In fact, werethe sig- -modulation. Were the signal current of complex form thefrequency of the carrier wave is still modulated in both forms ofmodulation. Phase modulation is merely that form of frequency modulationin which the amplitude of the frequency shift of the carrier is directlyproportional to the modulation frequency as indicated by the abovemathematical relationship m=A(0/I-h Of course, instead of covering aportion of the screen with opaque material and leaving the restcompletely visible the entire screen may be covered with material havinga varying degree of translucence, the variations being in accordancewith the sine and cosine function. Such a coating for the end of thecathode ray tube is indicated in Fig. 2 in which the shaded areas 33represent portions of minimum visibility of the screen and the unshadedportions represent the portions of greater visibility of the screen, thetranslucence of the ma- .grids as represented at 34- and 35 in Fig. 4may be positioned across the end of the cathode ray device. These gridsmay comprise parallel conintervals across the end of the cathode raytube, the conductors 36 of one grid 34 being displaced from theconductors 31 of the other grid 35 by a quarter of the distance betweenthe conductors.

The conductors 36 are connected together and to conductor 38 leading tothe signal amplifier 40. Similarly conductors 31 are connected togetherand to conductor 39 leading to signal amplifier 4|.

The vertical line transcribed by the cathode ray beam, in the absence ofsignal currents, thus cuts across the two' grids at a predeterminedpoint indicated by the vertical line 42, and when deflectedlongitudinally of the two grids by signal currents causes varyingcharges to be impressed upon the'conductors 38 and 33. That is, thecharge on either conductor is a maximum when the beam falls directlyupon one of the conductors 36 or 31. This charge then graduallydiminishes to a minimum when the beam -falls midway between suchconductors. lItthen increases as the beam approaches the next conductor.39 varies sinusoidally. Due to the relative displacement of theconductors 31 relative to conductors 36 along the lengths of the grids,the

Thus the charge on conductors 38 and While I have-s o n u ia two' waysof producing the two-currentscosflt andysin (m :sin t), it will .be thatany means capable of. producing 'suclifllcurrents .withthe sinusoidalvariations therein, displaced in phase by 90 degreesm ay be employ vicesmay be devised.

Y Thus, while I have shown 'only particular embodlments of my inventionit of course, "be understood that I do not wish to be limited there-.if'Many such de- .to since many'modifications' may be madenand Icontemplate by the appended claims to-cover any such modifications asfall within the true spirit and scope of my invention. Y Y

What I claim as new andidesire to secureby Letters Patent of the UnitedStates, is: V

1. In combination, two sources of carrier wave aaeaaoe current, means totranslate variations of said signal current into two light variations,eachvarying as sinusoidal functions of said signal oscillations of thesam frequency one of said sources being displaced in phase from theother by substantially ninety degrees, a source of signal current, meansfor modulating the oscillations of each of said carrier wave sources inac-' cordance with a sinusoidal function of said signal current, the.modulation of one of said carrier wave sources being displaced in phaseby ninety degrees from that of the other, and additively combining theoutputs from, said modulating means.

2. In combination, two sources of carrier wave oscillations of the samefrequency displaced in phase by ninety degrees, a source of signaloscillations, means for producing a current varying as a cosine functionof said signal oscillations and another current varyingas a sinefunction of said signal oscillations, means for modulating that one ofsaid carrier wave oscillations having the leading phase with that one ofthe signal oscillations having the lagging phase, and means formodulating the other source of carrier wave oscillations with theremaining source of signal oscillations, and means for combining. theoutputs from said modulating means.

3. The method of producing a carrier wave having'its frequency modulatedin accordance with a signal current which comprises amplitude modulatingtwo carrier waves of the same frequency and displaced in phase by ninetydegrees by currents varying as sinusoidal functions of said signalcurrent, the phase of the current with which the carrier wave of leadingphaseis modulated lagging the phase of the current with which thecarrier wave of lagging phase is modulated, and combining said twoamplitude modulated carrier waves to produce the frequency modulatedcarrier wave.

4. In combination, a pair of amplitude modulators, a carrier wave sourceconnected to the inputs of said modulators, means to displace the phaseof oscillations supplied from said source to one of said modulators byninety degrees with respect to the phase of the oscillations supplied tothe other modulator, a source of signal current, means to supply currentto each of said modulators varying in accordance with a 'sinusoidalfunction of said signal currents, the

current, said variations being displaced in phase by substantiallyninety degrees, means for modulating the'leading one'of said carrierwaves in accordance with the lagging one of saidv light variations andfor modulating the remaining carrier wave in accordance with the otherof said rier wave aftermodulation.

7. In combination, a cathode ray tube having a fluorescent screen, meansfor producing a luminescent line across saidscreen, means to deflectsaid line along'said screen in accordance light variations, and means tocombine said "care I with signal current, said screen having'two sec- 1tions, means to render the luminescence of said screen visibleexteriorly of said cathode ray tube in amounts varying, sinusoidallyfrom prede- 'termined points simultaneously traversed by said sectionsof said screen exteriorly of said tube, two

.sources of carrier wave of the same frequency,

to deflect the electron beam of said tube in a straight line, a pair ofgrids, each grid havinga plurality of conductorsv connected together andextending parallel with said line, said conductors being spaced apart atregular intervals in a direction transverse of said beam, means todeflect said beam along said grids in accordance with signal currents;the various conductors of each,

grid being displaced from the conductors of the other grid byone-quarter of the distance between conductors, a pair of sources ofcarrier wave of the same frequency displaced in phase by ninety degrees,means to modulate one of said carrier] wave sources in accordance withthe electromotive force induced in one of said grids and to modulate theother of said sources in accordance with the electromotive force inducedin the other grid, and means to combine the output from said a carrierwave sources.

sinusoidal variations in said currents supplied to means to produce twocurrents, one varying as a 9. The method of modulating the frequency ofa carrier wave in accordance with a current represented by theexpression m sin pt, which comprises producingtwo currents representedby the expressions sin wtand cos wt, producing two additional currentsrepresented by the expressions cos (m sin t) and sin (m sin pt),amplitude modulating the current sin at with the current cost (m sin t),amplitude modulating the current cos at with thecurrent sin (m sin at)and combining the modulated currents so produced, where t=time Y =thesignal frequency f 'w=21r,f where is the frequency of the carrier wavem=a constant 10. The method of modulating the frequency of a carrierwave in accordancewith a current having a frequency II- which comprisesproducing two'currents represented by the expressions sin t andcos wt,producing two additional currents I represented by the expressions cos(m sin at) and sin (m sin t), amplitude modulating the current sin wtwith the currentcos (msin at) amplitude modulating the current cos ibtwith the current sin (m sin t) and combining the modulated currents soproduced, where I .t=time w=21rf where f is the frequency of the carrierwave m=-Aw/ where Aw is the maximum shift of the carrier frequencyproduced by the signal.

such that the-modulated waves when combined produce the desiredfrequency modulated wave.

12. The method of producing a carrier current having its frequencymodulated inaccord with a signal current which comprises producing twocarrier currents of the same frequency displaced in phase, modulatingthe amplitude of each carrier current in accord with a periodic functionof the signal current, the modulations of the two carrier currentshaving the same phase displacement one fromthe other as the two carriercurrents, and combining the resulting carrier currents.

13. The method of producing a high frequency carrier current having itsfrequency modulated in accord with the variations of a lower frequencysignal current which comprises producing two high frequency carriercurrents .of the same high frequency displaced inphase ninety degrees,modulating the amplitude of each of these high frequency carriercurrents in accord with periodic functions of said lower-frequencysignal current, said modulations of the two carrier currents having aninety degree phase displacement one from the other, and combining theresulting two carrier currents into a single carrier current whereby acarrier wave is produced having its frequency modulated in accord withsaid signal current.

14. The method of modulation which includes the steps of generating aconstant frequency carrier wave, producing signalling voltage ofarbitrary wave form, utilizing said signalling voltage to generate anauxiliary wave which is a simple. sinusoidal function of theinstantaneous value of said signalling voltage, and modulating said.

carrier wave in accordance with said auxiliary wave. I

15. Means for producing polyphase alternating currents having aninstantaneous frequency determined by the rate of change of a signalvolt age comprising, an energy carrying beam, means for deflecting saidbeam by an amount determined solely by the'instantaneous potential ofsaid signal, a succession of targets'in the path of

